The subject of the invention is the Near Field Optical Transmission Electron Emission Microscope, abbreviated NOTEM: an analytical imaging instrument dedicated to optical object inspection and observation with transmission electromagnetic radiation, that utilizes (for imaging, magnification and detection of the original photon transmission image) the secondary electrons generated by this image.
In its principle of operation, function and applications, the subject of invention concerns the area of investigation and imaging of:
1) objects in any physical state, located under vacuum as well as in the gas atmosphere under arbitrary pressure (also under overpressure), including insulators, semiconductors, conductors and superconductors, dielectrics, ferroelectrics, piezoelectrics, paraelectrics, diamagnetics, paramagnetics, ferromagnetics, ferrimagnetics, antiferromagnetics, chemical compounds and substances, minerals, organic and inorganic matter, living and nonliving matter, i.e. any biological material(also in vivo in its natural environment or exposed to any gas atmosphere) or fluids and p 2) processes (occurring in the above-mentioned objects), e.g. physical, chemical, physicochemical, electrochemical, temperature dependent, magnetic, electrical, etc. (also in real time),
with secondary electrons generated as a result of conversion of the object penetrating electromagnetic radiation, non-polarized or polarized linearly, circularly/elliptically, e.g.: synchrotron radiation, X-Ray, laser light, UV-light or visible light, etc.
The terms “photon” and “electron” occurring in the nomenclature of the subject of invention: “Near-field Optical Transmission Electron Emission Microscope” determine its simultaneous relationship to the two wide fields of imaging techniques: optical and electron microscopy.
Concerning the character of the photon interaction with the object of imaging in the context of the invention, the first of the afore mentioned fields of microscopy, namely the optical, will be reduced to the aspects of the near-field approximation only, originally described by E. H. Synge in 1928 and experimentally realized by E. Betzig et al.: “Breaking the diffraction barrier optical microscopy on a nanometric scale”, Science 251 (1991) 1468.
On the other hand, the object of invention in its electronoptical part belongs to the emissions- and transmissions-electron microscopy techniques utilizing ‘parallel imaging’, which is an intrinsic feature of the cathode lens objective, invented by E. Brueche in 1932 and developed by E. Bauer: “Cathode lens electron microscopy: past and future”, J. Phys.: Condens. Matter 21 (2009) 314.
The principle of optical imaging in the near-field regime has been originally applied in the Scanning Near-field Optical Microscope (SNOM), in which the light source, i.e. an optical fiber tip, is moved close above the object's surface at a distance shorter than the photon's wavelength, resulting in the lower lateral resolution.
The invention's utilization of the principle of the conversion of the transmission optical image into a correlated emission electron image in the object plane of the cathode lens objective determines its relation with the two types of electron microscopy: 1) TEM—Transmission Electron Microscopy and 2)EEM—Emission Electron Microscopy (PEEM—Photoemission Electron Microscopy, LEEM—Low Energy Electron Microscope).
In the Transmission Electron Microscopy TEM developed by Ernst Ruska in 1933, the object of imaging(a transparent foil) will be penetrated by the high energy electron beam, which implies its localization under vacuum: “Transmission Electron Microscopy”, D. B. Williams and C. B. Carter, 2009.
Because the penetration depth of the X-Ray in matter is significantly higher than the penetration depth of electrons, only the former (also a synchrotron radiation) has been utilized in many further instrumental developments of Transmission Optical Microscopy: “Projection X-Ray Microscopy” (Newberry, 1954), “Imaging X-Ray Microscopy” (Rudolf et al., 1984), “Scanning X-Ray Microscopy” (Horowitz and Howell, 1972) and “Contact Imaging” (Goby, 1913).
Since its introduction at the “Symposium on X-ray Microscopy and Microradiography” in 1956 by G. Möllenstedt and L. Y. Huang, followed by their article “Röntgen-Bildwandler-Mikroskopie” published in the next year in Zeitschrift für Physik, 149 1957 p. 225, the original concept of the conversion of the transmission optical image to the photoelectron image developed in the Institute of Physics of Gottfried Moellenstedt, has been the subject of many subsequent instrumental realizations.
A representative example of further developments is described in the article of F. Polack and S. Lowenthal in “Journal de Physique, Colloque C2, suppl. no. 2, Tome 45, Fevr. 84, 1984, p. c2-73”, which initiates the new class of electron microscopy: “Photoelectron X-Ray Microscopy”.
In their version of the transmission microscope the homogeneous, smooth and structureless 100 nm-500 nm thin kapton foil (used as an object holder) has been covered on the vacuum side by a photoemissive material in order to convert photons to electrons.
A further improvement from 1988, presented in the report: “First Images with the Soft X-Ray Image Converting Microscope at LURE”/X-Ray Microscopy II, Springer Series in Optical Sciences 56 1988 220, has resulted in the application of cesium covered gold foil instead of the previously used kapton foil.
Another version of the Photoemission Electron Microscope invented by H. Hirose in 1990 is known from the U.S. Pat. No. 5,045,696, in which an analyzed object is held by a holder that consists of a support silicon membrane (on the object side) and a photocathode layer attached to the opposite membrane surface (on the vacuum side).
In this realization the earlier applied kapton or gold foil has been replaced by a 100 nm thin, homogeneous, smooth and structureless doped silicon membrane as an object holder (covered on the vacuum side by cesium iodide) that separates the vacuum part of the instrument from the atmospheric air and is surrounded by the magnetic coil and equipped with a grid electrode in order to generate a photoelectron image.
In the subsequent concept of Sh. Ohsuka et al. from 1990 entitled: “X-ray image observing device” and published in U.S. Pat. No. 4,912,737, the X-Ray transmission microscope has been equipped with the magnetic lens system only (without any electrostatic extractor field).
After transmission through the investigated object and its homogenous, smooth and structureless holder membrane, the divergent X-Ray radiation enters the vacuum area at the photocathode layer, where the optical image evolves and the photoelectric process takes place, resulting in the conversion to the photoelectron image.
This image will be further magnified with the magnetic lens system (without an application of the electrostatic extractor field) and projected on the fluorescence screen.
Another version of the optical transmission electron microscope invented by Bi Yu in 2005 is known from the U.S. Pat. No. 7,006,741, in which the fiber optic taper has been adapted to convert the optical image into the photoelectron image.
In lieu of the homogenous, smooth and structureless object-holder membrane, a massive optical element constructed from a number of glass fibers in the form of a cone has been applied, with the narrow face exposed to the investigated object and the wide face covered by a photoemissive layer exposed to vacuum.
The photons of the magnified (by the fiber optic geometry) optical image generate, as a result of the photoelectric effect in this layer, a photoelectron image on the vacuum side.
The above-mentioned realizations have been extended in 2006 by S. Fujii et al. in a “X-Ray Microscope Apparatus” by the three aspects reported in the U.S. Pat. No. 7,039,157: a) an integrated X-Ray laser source, b) the electrostatic component of the field around the photocathode/object and c) a tilted image detector (referred to the radiation axis).
The second technology area that is relevant to the subject of invention refers to the Electron Microscopy based on the cathode lens objective (immersion objective): 1) E. Bauer'a: “Surface Microscopy with Low energy Electrons”, Springer Verlag, 2014, and 2) O. H. Grifith's and W. Engel's, “Historical perspective and current trends in emission microscopy, mirror electron microscopy and low energy electron microscopy”, Ultramicroscopy 36 (1991).
The advantage of the “cathode lens objective” based electron microscopy has been confirmed in many instrumental realizations: in the case of photoelectrons as a PEEM (Photoemission Electron Microscopy)—E. Brüche, in the case of slow electrons as a LEEM (Low Energy Electron Microscopy)—E. Bauer and SPLEEM (Spin Polarized LEEM)—K. Grzelakowski et al., JEEE Transactions on Magnetics, 30 6 (1994), as well as DEEM (Dual Emission Electron Microscope): K. Grzelakowski, Ultramicroscopy 130 (2013) 29.
Such a “cathode lens objective”—based PEEM (Photoemission Electron Microscope) with the transmission photon-electron image conversion has been reported in 1997 by R. N. Watts et al. in Rev. Sci. Instrum. 68 (1997) 3464 as a “High Resolution Image Converter For Soft X-Ray Microscopy”, which utilizes the earlier idea of the homogenous, smooth and structureless silicon nitride membrane covered (on the vacuum side) by the carbon buffer layer and cesium iodide as a photocathode.
Another analogous realization of the idea and its application is known as the X-Ray Transmission Electron Microscope from two publications of G. De Stasio et al., respectively: Rev. Sci. Instrum. 69 (1998) 3106 and Rev. Sci. Instrum., 71 (2000) 11.
Also the biological applications of the Photon Transmission Electron Microscope in the investigation of the copper nanoparticles in protein KLH1 are known, published by D. Panzer et al. in Eur. Biophys. J 38 (2008) 53: “Transmission photoemission electron microscopy for lateral mapping of the X-ray absorption structure of a metaloprotein in a liquid cell”.
A further electron microscopy area that is relevant to the subject of invention, refers to the object inspection of integrated circuits in lithographic techniques.
Such a mask inspection instrumentation for the imaging of the lithographic mask described in U.S. Pat. No. 6,002,740 by F. Cerrina and T. B. Lucatorto is based on the integration of the PEEM microscope into the production line just above the lithographic mask that is illuminated from the opposite side by X-Ray radiation.
Similar to the former solutions, the photon-electron converter consists of the photocathode as a phosphor-cesium iodide layer evaporated onto the homogenous, smooth and structureless 100 nm thin silicon nitride membrane.
A further instrumental realization in the field of photon transmission electron microscopy: “X-ray photoemission microscope for integrated devices”, which was also dedicated to the inspection of an integrated circuits, has been presented in 2014 by D. L. Adler in the US patent application No. 20140037052.
In this case the X-Ray radiation penetrates the inspected integrated device located outside the vacuum, propagating towards the homogenous, smooth and structureless converter inside the vacuum, separated from the object by the X-Ray transparent window.
Another variation of Adler's concept adapts the converter geometry of H. Hirose from 1990 (U.S. Pat. No. 5,045,696); however, the object of imaging is located on the air side, which enables its shift and adjustment.
In a further variation of his concept, D. L. Adler has proposed using homogenous and structureless beryllium-or diamond photocathode in lieu of the previous Hirose's silicon nitride photocathode membrane.
In the third variation, the photocathode has been evaporated directly onto the inspected integrated device.